1. Field of the Invention
The present invention relates to an image output apparatus, an image output method and an image output program product for expressing the shading of an output image using a line screen.
2. Description of the Related Art
In image output apparatuses such as image forming apparatuses and image display apparatuses, there is generally used a method for expressing the shading of an output image using a line screen.
There is also known a dither method for expressing the shading by increasing or decreasing the number and/or size of dots. Particularly, a method for generating a stripe pattern and expressing multi-gradation by changing the number and/or width of strips is called a dither method using a line screen (FIGS. 1A, 1B: described in detail later).
Generally, in a color image forming apparatus, images are formed using four color toners of C (cyan), M (magenta), Y (yellow) and K (black). Thus, upon displaying an intermediate color, the intermediate color is expressed by the dither method using a line screen. At this time, an intermediate graduation of the line screen is expressed for each toner color, and the intermediate color is expressed by superimposition printing of the respective toner colors.
In this case, each plane (toner output plane) has a different angle (screen angle) as shown in FIG. 3 lest a moire pattern should be formed by toner stripe patterns to be superimposed. The respective screen angles are set different, e.g. 75° for magenta, 45° for black, 15° for cyan and 0° for yellow.
However, upon printing by the dither method using a line screen, if angles of thin line segments constituting the stripe pattern and dot forming angles (screen angles) in the dither method approximate, the unevenness (jaggies) of the lines segments stands out.
FIG. 1A shows an image of a character “RE” in Japanese katakana (character similar to “V” in alphabet) in a case where the angle of a line segment of an oblique stroke P1 of “RE” approximates to a screen angle upon expressing this character “RE” similar to “V” using a mono-color line screen. FIG. 1B shows an example in which the angle of a line segment of an oblique stroke P2 of “RE” largely differ from the screen angle. In FIGS. 1A and 1B, the size of pixel points is enlarged so that the state of the line screen can be easily understood. If FIGS. 1A and 1B are seen from a more distant point, the appearance of jaggies making the unevenness of the outline of the character is more noticeable in the oblique stroke P1 of “RE” in FIG. 1A as compared to the oblique stroke P2 of “RE” in FIG. 1B.